High speed common emitter switch



Dec. 28, 1965 w. A. LACHER 3,225,566

HIGH SPEED COMMON EMITTER SWITCH Filed Dec. 22, 1961 +v -v 62 e5 OUTPUT 2 e5 52 F/g. I" 5| OUTPUT 1 1 52 25 u INPUT Fig. /A 62 v 65 g 55 fi f OUTPUT 2 OUTPUT 1 ilg- 2 INVENTOR.

WILLIAM A. LACHER HEW AGENT United States Patent Ofiice 3,226,566 Patented Dec. 28',- 1965' 3,226,566 HIGH SPEED COMMON EMKTTER SWHTCH William A. Lacher, North Wales, Pa, assignor to Burroughs Corporation, Detroit, Mich, a corporation of Michigan Filed Dec. 22, 1961, Ser. No. 161,505 13 Claims. (Ci. 30788.5)

This invention relates to a high speed common emitter switch and more particularly to a high speed common emitter switch having a feedback network between the collector and base terminals that enables the transistor to be rapidly switched into the ON condition.

Common emitter switches are well known in the art. As generally used, the OFF condition, i.e., the transistor is non-conducting, of the common emitter switch is arbitrarily designated as a ONE or a ZERO. The ON condition, i.e., the transistor is saturated, is designated as a ZERO if the OFF condition is designated as a Conversely, if the ON condition is designated as a ZERO, the OFF condition will be designated as a ONE. Many applications of the common emitter switch require that the switch be capable of being very rapidly switched from the OFF to the ON condition and rapidly back to the OFF condition; Many circuit techniques have been devised to increase the speed of switching common emitter circuits from the ON to the OFF condition by controlling the degree of collector current saturation in the ON condition. Currently, improved transistors are available in which hole storage delay is minimal and switching time is not decreased by preventing the transistor from entering the saturation region during the ON condition. However, the delay in switching the common emitter switch from the OFF (non-conducting) to the ON (conducting) condition still limits the switching time of the common emitter circuit.

Accordingly an object of this invention is to decrease the time required to switch a common emitter circuit from the non-saturated condition to the saturated condition.

Another object of this invention is to provide two predictable and discrete voltage level outputs from a common emitter switch that are independent of transistor beta.

Still another object of this invention is to improve transistor circuits.

These and other objects of this invention are accomplished by providing a discontinuous feedback path in a common emitter circuit. The feedback operates during the OFF condition to enable the transistor to conduct slightly. 7 Consequently, the condition originally specified as the OFF condition is replaced by a condition of conduction in the active region of the transistor, which will be hereinafter referred to as the ACTIVE condition. The slight conduction of the transistor enables it to be switched rapidly into the ON condition. This switching time is less than the time required to switch the transistor from a non-conducting OFF condition into the ON condition. Accordingly, the switching time of the common emitter switch is improved. During the ON condition there is no feedback, i.e., the feedback path is rendered inoperative.

More specifically, the feedback path comprises a voltage regulating device such as a back biased Zener diode serially connected to a switching diode. This series circuit is connected between the collector and the base of the common emitter switch. During the ACTIVE condition, the feedback path from the collector, through the voltage regulating device and diode to the base of the switch enables a small amount of base current to flow. This slight base current primes the transistor and enables it to be rapidly switched into the ON (saturated) con-j dition. When the transistor is switched into the ON condition the switching diode is rendered non-conducting which prevents any further feedback current to the base of the transistor. The voltage regulating device always conducts and results in two discrete output level voltages. The voltage levels of these output signals do not change when it becomes necessary to replace (because of age or detects) the circuit transistor with another of the same type. t

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. 1 shows a common emitter switch" containing a preferred embodiment of the present invention; 7

FIG. 1A shows a modification of the circuit shown in FIG. 1;

FIG. 2 shows a common emitter flip-flop containing a preferred embodiment of the present invention.

Referring now to the drawings there is shown in FIG. 1 a common emitter switch comprising a semiconductor device such as PNP transistor 11 having an emitter 21, base 22, and collector 23 electrodes. The control electrode or base 22 is coupled to an input terminal 61' by way of a parallel RC network comprising a resistor 33 and a capacitor 41. It is to be understood that the input terminal 61 is connected to a suitable source of negative input pulses. Such pulse inputs are well known in the art and are derived from a wide variety of circuits. Resistor 33 is a current limiting resistor that limits the base current when a signal is applied to the input terminal 61'. The capacitor 41 is a speed up capacitor, which is well known in the art, that counterbalances the stray capacitance (not shown) of the transistor 11. The size of capacitor 41 is determined by the amount of stray capacitance of the transistor 11.

The emitter electrode is returned to a source of reference potential such as ground. The collector electrode is returned to a terminal 63 by way of an impedance element such as resistor 31. The junction of the collector electrode and the resistor 31 provides a first output terminal 6 4. The collector electrode is also coupled to a terminal 62 by way of anelectronic'volt'age regulating device such as a Zener diode 51 and an impedance element such as a resistor 32. A relatively negative potential is applied to terminal 63 and a relatively positive potential is applied to terminal 62. These potentials may be supplied by separate potential sources or a single potential source having, a plurality of relative potential levels. A unidirectional current switching device such as a semiconductor switching diode 52 is coupled between the base 22' and the junction of the resistor 32 with the Zener diode 51. The unidirectional current device 52 is always poled in the same direction as the diode created by the base and emitter electrodes of the transistor. It is to be understood that any common diode may be used in the practice of this invention and that a semiconductor switching diode is preferred only because it can be rapidly switched between a conducting and non-conducting state. The junction of the switching diode 52, Zener diode 51, and resistor 32 provides a. second output terminal 65.

The operation of the circuit shown in FIG. 1 is such that current will flow from the terminal 62. through resistor 32, through Zener diode 51 in the reverse direction, resistor 31, to the terminal 63 regardless of the state of conduction of the transistor 11. The Zener diode 51 is back biased, i.e., it operates in the; avalanche region. The magnitude of the positive potential applied to terminal 62, the negative potential applied to terminal 63, the resistor" 32, and the resistor 31 are chosensuch that the proper operating potential always appears on the collector 23. For example, assume that the magnitude of the positive potential on terminal 62 is equal to the magnitude of the negative potential on terminal 63 and that the transistor 11 is not conducting. The magnitude of resistor 32 and they impedance of the Zener diode 51 will be larger than the magnitude of the resistor 31 which results in the collector 23 being properly biased at a negative potential because the larger voltage drop is across the combination of the Zener diode and resistor 32. When the transistor 11 is rendered conducting the potential at the collector 23 will become less negative.

When no input signal is applied to the input terminal 61 no voltage is available to produce base 22 current and in an ordinary commonemitter switch no base current would flow and transistor 11 would be non-conducting. However, reference to FIG. 1 shows that in the absence of a negative input signal on the input terminal 6-1, the collector 23 potential goes negative towards the value of the negative potential seen on terminal 63. This negative potential on the collector 23 enables current to flow through the back biased Zener diode 51, the switching diode 52, and the base 22 to emitter 21 diode of transistor 11. This feedback starts base current flowing turning on transistor 11, which in turn starts the collector 23 potential going towards ground. This continues until the secondoutput terminal 65 nears ground potential at which time an ACTIVE equilibrium condition is reached. In the' equilibrium condition the second output terminal 65 is slightly more negative than ground potential which enables a" small forward current to flow through the switching diode 52 and the base to emitter diode of the transistor 11. The potential on the first output terminal 64 is morenegative than the potential seen on the second output terminal 65 by an amount equal to the constant voltage drop across the Zener diode 51. Therefore, in the AcTlVE condition'two distinct voltage level outputs are available.

, When a negative input pulse is applied to terminal 61, the transistor 11 is conducting slightly asexplained above, and transistor 11 will be very rapidly. switched into the ON or saturation region. When the transistor 11 is conducting heavily the collector 23 potential, which is also the first output terminal potential, approaches ground and the potential on the second output terminal is more positive than this by an amount equal to the constant voltage drop across the Zener diode 51. The Zener diode is chosen such that in the ON condition a suflicient constant voltage is dropped across it to make the potential on the second output terminal 65 positive. positive potential on the cathode back biases the switching diode 52 rendering it non-conducting. With switching diode 52 not conducting, the feedback path is no longer active. Because diode 52 is a switching diode it can be rapidly switched back and forth between a conducting and non-conducting state.

Accordingly, in the ON condition the collector potential is approximately at ground potential and this potential is seen on the first output terminal 64. The second output terminal 65 is at a more positive potential than the first output terminal 64 by an amount equal to the voltage drop-across the Zener diode 51.- It is clear, therefore, that the present invention provi es two outp t sig- T his nals each having different and determinable voltage levels. These output levels can be accurately determined because the collector potential in the ON condition is a function of the collector 23 to emitter 21 voltage of the type transistor being used. This transistor parameter is closely controlled by transistor manufacturers and is independent of transistor beta. The voltage drop across the Zener diode 11 is constant thereby causing the potential on the second output terminal 65 to be more positive than the potential on the first output terminal by a known value. In the ACTIVE condition the collector 23 potential ,is the summation of the voltage drops across the Zener diode 51, the switching diode 52 and the base 22 to emitter 21 diode of the transistor. The parameters of the aforementioned devices that determine the magnitude of these voltage drops are closely controlled by the manu facturers. Therefore, the output voltage levels of the two outputs can be determined accurately.

The invention described herein is used to reduce turn on time and is not intended to reduce the switching time due to hole storage. Presently, transistors are available in which hole storage is reduced to a minimum thereby making turn off time negligible. However, if desirable, the present invention can be combined with back clamping and other non-saturating techniques to produce a common emitter circuit which will operate entirely in the active region. 1

It is to be understood that other constant voltage devices may be used in the practice of this invention other than the Zener diode 51. Many electronic devices are available that produce a constant voltage drop. FIG. 1A shows a modification of FIG. 1 in which the Zener diode 51 is replaced by a cold cathode gaseous-glow tube 53. Such a tube has a larger magnitude of constant voltage drop across it than do most common Zener diodes which results in a smaller choice of suitable transistors that may be used. Also the cold cathode gaseous-glow tube is not backbiased in the circuit as is the Zener diode. The Zener diode is necessarily back biased because it only produces a constant voltage drop when it is operated in the avalanche region. The Zener diode 51 may also be replaced with other electronic constant voltage devices such as a neon bulb.

The common emitter switch maybe utilized by itself as a switch or it may be combined with other common emitter switches to form flip-flops, counters, shift registers, distributors, etc. It is to be understood that the present invention is not limited to a common emitter switch but includes all the circuits which may be formed by using the present invention. known flip-flop comprising two common emitter switches containing the present invention and is illustrative of many ways in which the improved common emitter switch, comprising this invention, may be utilized.

It will be obvious to those skilled in the art that the PNP transistor .11 can be replaced'with a NPN transistor and the circuit would operate equally as well as long as the polarity of the potential sources are reversed and the diode 52 and constant voltage devices 51 or 53 are reversed.

Obviously many modifications and variations of the present invention are possible. in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be FIG. 2 shows a well.

3 current device, said unidirectional current device connected between said voltage regulating means and said base electrode in a polarized direction such that upon activation a forward bias is applied to the junction formed by said base and emitter electrodes of said transistor to thereby enable said transistor to be maintained in an active conducting region between input pulses.

2. In a common emitter switch circuit of the class wherein the emitter electrode of a transistor is directly connected to a reference potential, the base electrode is coupled to a source of input pulses, and the collector electrode is returned to a source of potential through an impedance, automatically discontinuous feedback means comprising: voltage regulating means having at least a first and a second terminal, a unidirectional current device having at least a first and a second terminal, said first terminal of said regulating means connected to said collector electrode, said second terminal of said regulating means connected to one of said terminals of said unidirectional current device, said other terminal of said unidirectional current device connected to said base electrode, the unidirectional current device being interconnected in a polarized direction to automatically apply a forward-bias voltage to said transistor base electrode at the termination of one input pulse and automatically remove said forward-bias voltage upon the initiation of the next successive pulse, and a resistor connected between a further potential source and the common connection between said voltage regulating means and said unidirectional current device, said resistor and further potential source providing proper bias operating conditions for said voltage regulating means, whereby a discontinuous bias voltage is applied to reduce the turn-on time of said switch circuit.

3. The combination defined in claim 2 wherein said voltage regulating means is a back biased Zener diode.

4. The combination defined in claim 2 wherein said unidirectional current device is a semiconductor switching diode.

5. The combination defined in claim 2 wherein said voltage regulating means is a cold cathode gaseous-glow tube.

6. An electrical circuit comprising: a semiconductor device having at least an emitter, a collector and a base electrode, a first and a second source of potential, a first impedance element connected between said first potential source and said collector electrode, a second impedance element having one of its ends connected to said second source of potential, a unidirectional current device having one of its ends connected to the other end of said second impedance element and its other end connected to said base electrode, a voltage regulating means connected between the junction of said collector electrode with said first impedance element and the junction of said unidirectional current device with said second impedance element, said current device being interconnected in a polarized direction to automatically apply a forward-bias voltage to the junction of the emitter-base electrodes of said semiconductor device, the application of said forward-bias voltage by said current device being automatically discontinued during the period the collector junction of said semiconductor device is forward-biased.

7. The combination defined in claim 6, wherein first output means is coupled to the junction of said collector electrode with said first impedance element.

8. The combination defined in claim 7 wherein second output means is coupled to the junction of said second impedance element with said unidirectional current device and said voltage regulating means.

9. A common emitter switch circuit comprising: a transistor having an emitter, a base, and a collector electrode, a first and a second source of potential, a source of reference potential, said emitter electrode connected to said reference potential, a first resistor connected between said first source of potential and said collector electrode,

6 said base electrode being adapted to receive input pulses, a second resistor having first and second terminals, a semiconductor switching diode having first and second terminals, said second resistor having its first terminal connected to said second source of potential andits second terminal connected to said first terminal of said semiconductor diode, said semiconductor diode having its second terminal connected to said base electrode, a re verse-biased Zener diode connected between said collector electrode and the junction of said second resistor with said semiconductor diode, said diode being forward-biased between, and reverse-biased during, the application of each of said input pulses whereby said transistor is rendered active in the absence of said input pulses to thereby reduce the time required to saturate said transistor by the presence of an input pulse.

10. A common emitter switch circuit comprising a transistor having a base, emitter and collector electrode, voltage divider means including a two-terminaled voltage regulating device in a series circuit connected between two levels of operating voltage potential, means connecting said collector electrode to one terminal of said voltage regulating device, a semiconductor diode connected from the second terminal of said voltage regulating device to said base electrode, means connecting said emitter electrode to a reference potential intermediate said two levels of operating voltage potential, said switch circuit including said voltage divider means arranged to provide a forward bias to said semiconductor diode to normally cause a priming current to flow through the emitter-tobase diode of said transistor and through said semiconductor diode to thereby provide a switching circuit having improved turn-on characteristics.

11. A common emitter switch circuit as defined in claim 10 wherein the two levels of operating voltage potential having the voltage divider connected therebetween are first and second constant voltage sources of direct current serially interconnected to provide two levels of potential having opposite polarities.

12. A continuously conducting transistor switching circuit having an input means to receive saturation-causing input pulses and at least two output means each having switched voltage levels which are independent of transistor gain characteristics, said switching circuit comprising a transistor amplfying means having input and output terminals, a constant voltage means connected to said output terminal, a normally closed unidirectionally conducting disconnect semiconductor means connected from said constant voltage means to said input terminal, said constant voltage means and said disconnect means being conductively connected to bias said transistor in its active conduction region in the absence of said saturation-causing input pulses and operatively responsive to their presence to disconnect said bias from the input terminal whereby said transistor amplifying means has an improved turn-on time permitting a rapid saturation.

13. A circuit comprising a transistor connected in a common emitter configuration with its emitter electrode connected to a reference potential, its collector electrode connected through a first impedance element to a fixed voltage level of a first polarity and its base electrode connec-ted to a source of saturation-causing input pulses, a voltage regulating means connected from said collector electrode through a second impedance element to a fixed voltage level of a second polarity, the junctions formed by said regulating means with said second impedance element and said collector electrode respectively creating first and second output terminals, and a diode device connected between said first output terminal and the base electrode of said transistor, said voltage regulating means providing a bias level to activate said diode device and said transistor in the absence of an input pulse from said source and a reverse-bias level to inactivate said diode device in the presence of a saturation-causing input pulse to thereby provide a transistor switching circuit with an 2,504,908 4/1950 Tryon 323-16 2,990,478 6/1961 Scarbrough 307-88.5 3,105,159 9/1963 Ditkofsky 30788.5

8 FOREIGN PATENTS 215,148 11/1957 Australia.

OTHER REFERENCES LBLM. Tech. Discl Bulletin, vol. 2, N0. 6, April 1960, pages 89 and 90.

ARTHUR GAUSS, Primary Examiner. 

12. A CONTINOUSLY CONDUCTING TRANSISTOR SWITCHING CIRCUIT HAVING AN INPUT MEANS TO RECEIVE SATURATION-CAUSING INPUT PULSES AND AT LEAST TWO OUTPUT MEANS EACH HAVING SWITCHED VOLTAGE LEVELS WHICH ARE INDEPENDENT OF TRANSISTOR GAIN CHARACTERISTICS, SAID SWITCHING CIRCUIT COMPRISING A TRANSISTOR AMPLIFYING MEANS HAVING INPUT AND OUTPUT TERINALS, A CONSTANT VOLTAGE MEANS CONNECTED TO SAID OUTPUT TERMINAL, A NORMALLY CLOSED UNIDRECTIONALLY CONDUCTING DISCONNECT SEMICONDUCTOR MEANS CONNECTED FROM SAID CONSTANT VOLTAGE MEANS TO SAID INPUT TERMINAL, SAID CONSTANT VOLTAGE MEANS AND SAID DISCONNECT MEANS BEING CONDUCTIVELY CONNECTED TO BIAS SAID TRANSISTOR IN ITS ACTIVE CONDUCTION REGION IN THE ABSENCE OF SAID SATURATION-CAUSING INPUT PULSES AND OPERATIVELY RESPONSIVE TO THEIR PRESENCE TO DISCONNECT SAID BIAS FROM THE INPUT TERMINAL WHEREBY SAID TRANSISTOR AMPLIFYING MEANS HAS AN IMPROVED TURN-ON-TIME PERMITTING A RAPID SATURATION. 